Nigel is correct - post 1884 the index to marriages is on microfilm but as each page contains only 2 or 3 entries Brian Lawson is in the process of reversing this index to give marriages but no details other than names of partners (usually census info fills in the rest)

Pre 1884 then unless one of the relatively few dissenter, civil or Roman Catholic marriages then you will get the information from the filmed 1911 transcript of the parish registers

thus paying for a certificate gives you the parents names post 1884 and of course may be less hassle or faster than ordering a microfilm (though I understood there were some films covering upto 1911 but here I may be confused as all my research has been done in Manx Museum and generally pre 1880)